Advances in biomedical research, medical diagnosis, prognosis, monitoring and treatment selection, bioterrorism detection, and other fields involving the analysis of multiple samples of low volume and concentration of analytes have led to development of sample analysis systems capable of sensitively detecting particles in a sample at ever-decreasing concentrations. U.S. Pat. Nos. 4,793,705 and 5,209,834 describe previous systems in which extremely sensitive detection has been achieved. The present invention provides further development in this field.